To the chloroform solution (20 ml) of H-Tyr(tBu)-Leu-OBzl was added HOBt (41 mg, 0.30 mmol), Fmoc-Glu(OtBu)-OH (468 mg, 1.10 mmol) and EDC.HCl (232 mg, 1.21 mmol) were added under ice-cooling, and the mixture was stirred at room temperature overnight. After completion of the reaction, to this solution were added under ice-cooling thiomalic acid (450 mg, 3.00 mmol) and DBU (1.49 mL, 10.00 mmol), and the mixture was stirred at room temperature for 3 hr. After completion of the reaction, to this solution was added dropwise under ice-cooling a mixture of MsOH (0.23 mL, 3.60 mmol) and chloroform (2.3 mL), chloroform (10 ml) and 20% brine (20 ml) were added to give an organic layer and an aqueous layer. Then, the aforementioned pH=9.0 washing was performed three times. The obtained organic layer was washed three times with 10% aqueous sodium carbonate solution (20 ml) and twice with 20% brine (20 ml) with stirring at room temperature, and the organic layer and the aqueous layer were partitioned. The obtained organic layer was dried over sodium sulfate and filtered to give a chloroform solution (30 ml) of H-Glu(OtBu)-Tyr(tBu)-Leu-OBzl. This chloroform solution was directly used in the next step.
